I would imagine that agricultural jobs would be low-risk for those types of employers since those with criminal records are unlikely to be interacting with customers or exposed to high value inventory. Both of which are characteristics of a job that would give employers pause about hiring someone with a criminal record.
A similar thing is happening here in my home state in Australia. The government introduced a new tax on the backpackers that traditionally filled out the fruit-picking workforce. The fruit-pickers cried foul about how the government was now strangling them... but this story has a twist: the fruit-pickers have been treating the backpackers like shit, withholding pay, for enough years for word to get around that it was no longer worth doing.
In short, local residents don't want to do short-term seasonal work under poor working conditions - unskilled agricultural jobs really suck. Not to mention that you're not going to draw city-dwellers out to the farm if they're poor enough to be attracted to that kind of work - who is going to give up their home for short-term work? Only those folks who don't have any roots put down; that is, people who are already travellers of some kind.
Longer-term agricultural work can attract people more easily, since they'll have an ongoing income and can put roots down.